This project will utilize laser Doppler velocimetry as a detector of light scattered from individual small particles and macromolecules in suspension. The laser technique offers a significant improvement in the signal-to-noise ratio of light scatter measurements. This instrument is capable, in principle, of detecting single particles of a few nanometer radius. By counting and sizing individual particles, an absolute determination of sample mass in a suspension is obtained. Potential applications of the instrument include counting and sizing individual viruses, subcellular organelles, and cellular macromolecules--including individual chromosomes. The instrument may also find application in the nephelometric analysis of submicroliter clinical samples. The determination of pediatric patient blood serum protein by immunoprecipitation assay is a potential clinical analysis.